querfeldein means “diagonally across the terrain / cross-country” or something, but here I think it is used not in a literal but in a figurative sense;

I think you have to look at the whole sentence “alles dabei, so einmal querfeldein”, which literally means “all included, like once diagonally across the terrain”, which in a figurative sense means that they produce pretty much everything that can be grown – this is also shown in the list of different produces just before that phrase; the phrase therefore acts to shorten the list and to say that they have/do a bit of everything.

- rather than across the board.

so einmal querfeldein : fairly and squarely pitched (right) across the pasture.

I think you need to get away from the literal meaning, as some of the previous posters/commentators have pointed out, too. In addition, "querfeldein" is used in quite a strange way in your sentence. I'd rather think of "(einmal) quer durch den Gemüsegarten" - see https://www.duden.de/rechtschreibung/Gemuesegarten

I get the impression the word is used 'improperly', the point being to play on the word 'Feld' while containing the meaning of 'everything', 'across the board'.

An 'expert across the field' (in whatever field) is an expert in everything involved in the field, as opposed to a 'super-expert' with expertise in a more detailed aspect.

An agricultural 'super-expert' might produce carrots, OR spuds, OR onions. An agricultural 'expert across the field' might produce all of those, and more.

'Across the field' might seem a bit laboured, but is not 'querfeldein' too?